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Prior exercise speeds pulmonary O₂ uptake kinetics by increases in both local muscle O₂ availability and O₂ utilization

¹Canadian Centre for Activity and Aging, ²School of Kinesiology; and ³Department of Physiology and Pharmacology, The University of Western Ontario, London, Ontario, Canada

Submitted 21 September 2006 ; accepted in final form 4 May 2007

The effect of prior exercise on pulmonary O₂ uptake (O_2p), leg blood flow (LBF), and muscle deoxygenation at the onset of heavy-intensity alternate-leg knee-extension (KE) exercise was examined. Seven subjects [27 (5) yr; mean (SD)] performed step transitions (n = 3; 8 min) from passive KE following no warm-up (HVY 1) and heavy-intensity (50%, 8 min; HVY 2) KE exercise. O_2p was measured breath-by-breath; LBF was measured by Doppler ultrasound at the femoral artery; and oxy (O₂Hb)-, deoxy (HHb)-, and total (Hb_tot) hemoglobin/myoglobin of the vastus lateralis muscle were measured continuously by near-infrared spectroscopy (NIRS; Hamamatsu NIRO-300). Phase 2 O_2p, LBF, and HHb data were fit with a monoexponential model. The time delay (TD) from exercise onset to an increase in HHb was also determined and an HHb effective time constant (HHb – MRT = TD + ) was calculated. Prior heavy-intensity exercise resulted in a speeding (P < 0.05) of phase 2 O_2p kinetics [HVY 1: 42 s (6); HVY 2: 37 s (8)], with no change in the phase 2 amplitude [HVY 1: 1.43 l/min (0.21); HVY 2: 1.48 l/min (0.21)] or amplitude of the O_2p slow component [HVY 1: 0.18 l/min (0.08); HVY 2: 0.18 l/min (0.09)]. O₂Hb and Hb_tot were elevated throughout the on-transient following prior heavy-intensity exercise. The LBF [HVY 1: 39 s (7); HVY 2: 47 s (21); P = 0.48] and HHb-MRT [HVY 1: 23 s (4); HVY 2: 21 s (7); P = 0.63] were unaffected by prior exercise. However, the increase in HHb [HVY 1: 21 µM (10); HVY 2: 25 µM (10); P < 0.001] and the HHb-to-O_2p ratio [(HHb/O_2p) HVY 1: 14 µM·l^–1·min^–1 (6); HVY 2: 17 µM·l^–1·min^–1 (5); P < 0.05] were greater following prior heavy-intensity exercise. These results suggest that the speeding of phase 2 O_2p was the result of both elevated local O₂ availability and greater O₂ extraction evidenced by the greater HHb amplitude and HHb/O_2p ratio following prior heavy-intensity exercise.

near-infrared spectroscopy; muscle O₂ utilization; muscle blood flow; O₂ kinetics; slow component

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